The present invention relates to linear polyethylenes obtained by catalytic (co)polymerisation of ethylene, optionally with xcex1-olefins.
Linear polyethylenes, such as high density polyethylene (HDPE) and linear low density polyethylene (LLDPE), can be endowed with specific properties through a suitable choice of polymerisation conditions and polymerisation catalysts, of which chromium oxide and metallocene catalysts are among the commercially most important. Chromium oxide catalysts are characterized in that their active sites have different ratios between propagation and chain transfer. Moreover, their active sites have different ratios between comonomer propagation and ethylene propagation ratios. As a result, the obtained polymer will have possible comonomers (side chains) unevenly distributed between the macromolecules. Consequently, the produced polymers will have a broad molecular weight distribution, and these polymers will have good processability properties. The metallocene catalysts, often called single site catalysts (SSC) because all their active sites are equal, produces polymers having a narrow molecular weight distribution and an even distribution of possible comonomers and side chains along the polymer backbone. These polymers have desired properties like high impact strength, good transparency, and high hot tack. The main disadvantage is their poor processability due to to the narrow molecular weight distribution. A combination of the properties of chromium oxide and metallocene polymers would be highly desireable for many end uses, in particular will polyethylenes having a controlled broad or bimodal molecular weight distribution be advantageous in polymer processing by extrusion. Such polyethylenes have traditionally been produced by consecutive polymerisations at different conditions in two reactors in a series. Another approach has been to perform the polymerisations in the presence of a modified catalyst, including the use of combinations of different catalysts. However, when combining different catalysts, they often have a tendency to inactivate each other.
U.S. Pat. No. 3,378,536 discloses a process for the polymerisation of ethylene by the use of a two-component catalyst system consisting of (a) chromium deposited on e.g. silica; and (b) chromium or vanadium arene where the arene is an aromatic, optionally substituted, C6 ring. The two catalyst components are preferably fed separately to the polymerisation reactor.
EP 088 562 discloses a modified polymerisation catalyst comprising a silica support with deposited chromium. Following oxidation in dry air, the chromium is modified by being contacted with a transition metal compound. The obtained polyethylenes have a substantial degree of branching and a medium to broad molecular weight distribution.
U.S. Pat. No. 5,330,950 and 5,408,015 relate to ethylene polymers having broad molecular weight distributions, obtained by the use of a catalytic mixture including a MgO-supported Ziegler catalyst and a chromium oxide catalyst.
EP 339571 discloses a process for producing polyethylenes having a broad molecular weight distribution by the use of a catalyst system consisting of: a catalyst component (A) comprising a silica support onto which there is deposited a titanium or chromium compound; a catalyst component (B) comprising a transition metal compound; and a catalyst component (C) which is an aluminoxane, e.g. MAO. The MAO and the metallocene are not impregnated into the catalyst support and therefore the catalyst will not be a true dual site catalyst. The two types of catalyst sites or precursor sites are not directly exposed to each other. There is no mention of any prereduction of Cr6+ to Cr2+. The polymerisations will result in polymers having an undesireable amount of low molecular weight polymers.
WO 96/14154 discloses a dual site catalyst comprising a calcined silica support impregnated with dibutylmagnesium, 1-butanol and TiCl4, and also impregnated with MAO and (BuCp)ZZrCl2. This catalyst, which contains both a non-metallocene and a metallocene transition metal component, is used in the polymerisation of ethylene in one single reactor to obtain polyethylenes having a bimodal molecular weight distribution.
The object of the present invention is to provide ethylene polymers having a controlled bimodal or broad molecular weight distribution. The polyethylenes are obtained by the use of a novel dual site catalyst that combines the features of both chromium oxide and metallocene catalysts. A remarkable advantage is that this catalyst produces chromium and metallocene polyethylenes simultaneously and mixed in one and the same polymer particle. The polymer particles as polymerised will when processed give a homogeneous polymer melt. With this novel dual site catalyst polyethylene resins for a broad application area can be produced. The resins have particularly good extrusion properties and are especially well suited for processing by film blowing and foaming, optionally for blow moulding and pipe extrusion.
The present invention thus provides a polyethylene produced by the homopolymerisation of ethylene or copolymerisation of ethylene with xcex1-olefins in the presence of a catalyst, characterized in that the fraction of low molecular weight polymers having MW less than 5000 g/mole is lower than 12% by weight of the produced polymer, and that the produced polyethylene has a measured melt strength expressed by a haul-off (HO) value satisfying the expression:
HO greater than K1xc2x7xcex70.05+K2,
wherein K1 is 9xc2x710xe2x88x925 and K2 is 0.4.
The polyethylenes are preferably obtained by polymerisation in a single polymerisation process in the presence of a particulate modified catalyst comprising:
a) a chromium-oxide catalyst comprising a chromium oxide mainly in a bivalent oxidation state combined with an inorganic support containing above 85% by weight of silica,
b) a transition metal compound comprising at least one cyclopentadienylic ring bonded to said transition metal, which cyclopentadienylic ring may contain hetero atoms, be unsubstituted or substituted, bonded to the transition metal through a bridge, optionally annealed to other substituted or unsubstituted ring structures, and if two cyclopentadienyl rings are present they may be bonded to each other through a bridge, and
c) a catalyst activator being an aluminoxane.